Rotating electric machines such as electric motors or generators generally have an annular stator defining a stator bore that houses a rotor.
The stator includes a plurality of laminations insulated from one another and defining axial slots.
Within the axial slots bars are housed (usually one or two bars but also more bars can be housed in each slot); the bars are connected at their ends to define stator windings.
The bars include a conductor and insulation around it.
The conductor is usually defined by a plurality of transposed copper strands, each insulated from the others.
The insulation is usually defined by a mica tape impregnated with a resin.
The bars are firmly fixed in the slots so as to reduce the vibrations, thus preventing insulation damages and electric discharges within the slots.
The dielectric performances of the insulation deteriorate during operation, so that the electric machine could have a reduction of the expected lifetime.
This ageing depends on the design and operating conditions, as well as on the environmental conditions.
For this reasons, in some cases refurbishment is needed.
Refurbishment can include stator and/or rotor repair or a full rewind (e.g. replacement of the stator bars to provide a new winding) and can lead to a general lifetime extension of the electric machine.
In order to carry out the refurbishment or rewind, it is often needed to remove the bars.
In order to remove the bars, the bar ends that protrude from the stator core (the so called end windings) are first cut and removed.
Then, also the bar parts housed in the slots is removed.
In many cases the bars are firmly fixed in the slots, therefore consistent efforts are required to remove them.
Fixation is particularly strong in case of stator manufactured according to the Global Vacuum Pressure Impregnation process.
The Global Vacuum Pressure Impregnation process consists of introducing the stator with the bars already inserted in the slots into a tank; then resin is introduced into the tank, such that the whole stator is impregnated with it.
In these cases extraction of the bars from the slots is often performed through hydraulic extraction tool.
This traditional extraction method has some drawbacks.
In fact, this method is often time consuming, because during extraction breakage of the components (such as wires) used for extraction can occur. This requires the replacement of the broken components and the positioning of the new components.